Handovers between different types of communications systems such as between a cellular mobile telephony network and a wireless LAN are called vertical handovers and study results thereof have been reported in papers, a few of which are cited below.
In one paper, for example, J. Inouye, J. Binkley, J. Watpole, “Dynamic Network Reconfiguration Support for Mobile Computers,” Proceedings of ACM/IEEE International Conference on Mobile Computing and Networking (Mobicom'97), Budapest, September 1997 (Non-patent document 1), an example of a networks system where a mobile host adaptively changes over between its network interfaces and transmission routes is discussed. According to this paper, an experiment using this networks system revealed that, when the mobile host changes over between an interface for a wired LAN and an interface for a wireless LAN, for example, its Internet Protocol (IP) address changes as the interface changes, and consequently, an application such as telnet cannot be continued. If, for example, the mobile station performs path rerouting from the wired LAN to the wireless LAN, it has to release a communication path set up, using its IP address within the wired LAN, and reconnect to the wireless LAN, using its IP address within the wireless LAN. The mobile station has to re-execute all applications running on it for new connection.
In another paper, for example, Marc Bechler, Hartmut Ritter, “A flexible Multiplexing Mechanism for Supporting Quality of Service in Mobile Environments,” Proceedings of the Hawaii International Conference on System Science, Maui, Hi., January 2001 (Non-patent document 2), examples of carrying out handovers between different types of communication systems such as between a wireless LAN and a cellular mobile telephony network, using Mobile IP technology, are discussed. In these examples, an IP packet into which an application's data is packaged for transmission to a particular destination is encapsulated into an IP packet with the IP address of a selected network device and then transmitted via the network device to the destination. Because the IP address of the IP packet of the application's data is unchanged, the application can continue while the handover takes place.
FIG. 1 shows an example of conventional wireless system architecture using the Mobile IP technology. Reference numeral 1 denotes a contents server, 2 denotes an IP network, 3 denotes a home agent of Mobile IP, and 8 denotes a mobile station. Reference numeral 4 denotes a foreign agent in a wireless LAN network, 5 denotes the wireless LAN network established by a wireless LAN service provider, and 9, 10, and 11 denote access points of the wireless LAN. Reference numeral 6 denotes a foreign agent in a 1xEvDO (cellular mobile telephony system) network, 7 denotes the 1xEvDO network established by a 1xEvDO service provider, and 12, 13, and 14 are access points of the 1xEvDO. Reference numeral 15 denotes a fixed network. Communication from the home agent to a foreign agent is performed through an “IP tunnel” formed by encapsulating an IP packet within an IP packet. The address of the end point of the IP tunnel is a Care-of address. A home address is an IP address assigned to the mobile station 8, which is independent of where the mobile station 8 is located and attached to the Internet. IP packets transmitted to the home address of the mobile station 8 are intercepted by the home agent and then forwarded to the Care-of address. The IP tunnel may be terminated at the mobile station 8 itself. The foreign agent or the mobile station 8 itself at the end point of the IP tunnel receives encapsulated datagrams and the foreign agent forwards decapsulated IP packets to the mobile station 8.
FIG. 2 illustrates an example of exchanging messages for a handover of the mobile station (MS) 8 in the system of FIG. 1. If the mobile station 8 is handed over from a wireless LAN access point to a 1xEvDO access point, the Source FA 16 is a foreign agent 4 in the wireless LAN system and the Target FA 17 is a foreign agent 6 in the 1xEvDO system. If the mobile station 8 is handed over from a 1xEvDO access point to a wireless LAN access point, the Source FA 16 is a foreign agent 6 in the 1xEvDO system and the Target FA 17 is a foreign agent 4 in the wireless LAN system. Assume that the mobile station 8 is downloading contents 20 from the contents server 1 via the foreign agent 16. When the mobile station 8 detects a new communication system and determines to perform a handover to that system, it transmits an Agent Solicitation 21 for receipt by any nearby foreign agent within the subnet of the new system to solicit transmission of an Agent Advertisement message. The foreign agent 17 transmits an Agent Advertisement 22 to announce its service over the subnet and, when the mobile station 8 receives this message, it determines in which network it is attached. Even when not receiving the Agent Solicitation 21, the foreign agent 17 periodically broadcasts the Agent Advertisement 22 over the subnet. The mobile station 8 transmits a Registration Request 23, 24 via the foreign agent 17 to the home agent 3 as a message to register its location with the home agent 3. The Registration Request 23 message includes the home address of the mobile station 8, the home agent address, and the address, care-of address of the foreign agent 17. Having received the Registration Request 23 message, the home agent 3 registers the care-of address of the mobile station 8 into a forwarded-to-address table. The home agent 3 transmits back a Registration Response 25 as a message to return the result of the mobile station's location registration to the foreign agent 17. The foreign agent 17 transmits back a Registration Response 26 as a message to return the result of the mobile station's location registration to the mobile station 8. The home agent 3 forwards contents 27 via the foreign agent 17. The foreign agent 17 receives IP encapsulated packets and delivers decapsulated IP payloads to the mobile station 8. Handover time depends on timing when the mobile station 8 receives the Agent Advertisement 22 and time required for rerouting on the IP layer is a few seconds.
FIG. 15 shows an encapsulation format example conforming to the IP encapsulation method as specified in RFC2003. The home agent 3 encapsulates an IP packet 151 consisting of an IP header 152 and an IP payload 153, as an IP payload 157, within an IP packet 155. In the IP header 152, the source address is the IP address of the contents server 1 and the destination address is the home address of the mobile station 8. In the IP header 156, the source address is the IP address of the home agent 3 and the destination address is the care-of address of the mobile station 8.
Because the encapsulation method in the above example encapsulates an IP packet within another IP packet, control information increases by the additional IP header and information transmission efficiency decreases. To reduce the increase in control information, another encapsulation method has been proposed as an Internet standard's track protocol. FIG. 14 shows an encapsulation format example conforming to the encapsulation method as specified in RFC2004. The home agent 3 constructs an IP packet 161 by adding an IP header 162 and an address 163 to the IP payload 153 of an IP packet 151. In the IP header 162, the source address is the IP address of the home agent 3 and the destination address of the care-of address of the mobile station 8. The address 163 is the home address of the mobile station 9 and a minimum of 12 bytes of information is added.
[Non-patent document 1] J. Inouye, J. Binkley, J. Watpole, “Dynamic Network Reconfiguration Support for Mobile Computers,” Proceedings of ACM/IEEE International Conference on Mobile Computing and Networking (Mobicom '97), Budapest, September 1997
[Non-patent document 2] Marc Bechler, Hartmut Ritter, “A flexible Multiplexing Mechanism for Supporting Quality of Service in Mobile Environments,” Proceedings of the Hawaii International Conference on System Science, Maui, Hi., January 2001